1. Field of the Invention
The present invention relates to a means for a temporal or a long storage of energy and a method for converting energy, and devices related to the same. More particularly, the present invention provides a means for storing energy and a device related thereto, said means and device being capable of accumulating energy at an extremely high density that has never been achieved to the present.
2. Description of the Prior Art
Though not always consciously enough, mankind has been confronted with the problem of energy storage since the ancient times. The problem of energy storage is closely related to the cyclic turn of day and night, or of winter and summer, which requires cyclic change in the supply and demand of energy.
For example, men have compensated for insufficient heat in cold nights and winters, i.e., when there is only an insufficient supply of solar energy, by converting the energy stored in nature into a convenient form for use by a chemical means such as combustion. More specifically, firewood and the charcoal resulting therefrom are, so to say, storage devices in the form of plant bodies which store the solar energy therein by photosynthesis. Coal and oil, whose consumption is highly increasing with modernization, can find their origin in ancient plants and animals. They store the ancient solar energy in the form of metamorphosed mortal remains of plants and animals.
Lead batteries are means for storing electric energy which have been invented in the nineteenth century, and secondary cells inclusive of lead batteries are all means for storing electric energy by chemical conversion.
Another type of energy storage comprises electrolyzing water and storing hydrogen gas which generates therefrom. Since hydrogen gas can be converted into harmless water upon combustion without producing any other unwanted substances, it has drawn attention recently as a clean energy source. This is a distinguished feature of this energy source as compared with the fossil fuels which are described hereinafter.
The means of energy storage mentioned above unanimously take advantage of a chemical reaction among molecules and atoms. Those means are characterized by that they can store energy at an extremely high density. In general, about 10.sup.23 atoms or molecules are present per 1 cm.sup.3. Since each of these atoms and molecules is capable of storing energy in an amount of from about 0.1 to 1 eV of energy per 1 atom or molecule, a theoretical calculation teaches that an energy of from 10.sup.9 to 10.sup.10 Joule (J) is stored in 1 m.sup.3 of those atoms and molecules. Practically, in fact, a liquid hydrogen can store energy at an amount of 1.02.times.10.sup.10 J/m.sup.3. In the case of secondary batteries, the accumulated energy density is about 10.sup.8 J/m.sup.3 for a lead battery. This low density is due to the presence of an electrolyte accounting for a large volume, besides the substance (electrode plates) which participates directly in storing energy.
The aforementioned means for storing energy by taking advantage of a chemical reaction are, however, not suited for drawing out energy in a large amount at one time. In the case of a secondary cell, for example, the maximum amount of energy which is drawn out depends on the surface reaction of the electrode. Other types of chemical energy storage may release the accumulated energy at once by causing an explosion, but it is nearly impossible to efficiently convert the energy into a convenient form such as electric energy.
In addition to the chemical means of storing energy as mentioned above, physical means for the same purpose are also used in various fields. A representative of such physical means comprises dams which are used at hydraulic power plants. A dam stores rainfall which has been produced by a solar energy and the like. In such a case, a potential energy is stored in the form of a large amount of water being stored in a dam. A pumped storage power plant draws up water using the excess electric power during night, and generates electric power by discharging the thus pumped up water in the daytime. In this case, the electric power in the night is stored in the form of a potential energy of the water. The technological concept of the pumped storage power plant resides in supplying energy in a flexible manner in accordance with the demand. The object of the present invention is based on a similar concept.
In the case of a dam having established at a distance, or fall, of 100 m from a power station, for example, the density of the energy stored by the water inside the dam is 10.sup.6 Joule per 1 m.sup.3 of water. If the fall were to be extended, the accumulated energy density would be increased in proportion to the fall.
A capacitor is also a physical means of storing energy, in:which the energy is stored in the form of a static energy. This means is characterized by that it allows drawing of high energy at one time. For example, in an extreme case, all the stored energy can be drawn out at a period as short as 10.sup.-9 seconds. However, in this case again, the density of the accumulated energy in general is not large.
In a capacitor, the energy is assumed to be stored by an electric field generated between the electrodes. In general, a substance would not withstand a limitless high electric field because a dielectric breakdown occurs upon application of an electric field exceeding a certain limit. This limiting electric field is 10.sup.8 V/m for a commonly known substance. Since E in general is in the range of from 10.sup.-11 to 10.sup.-9, the maximum density of the stored energy in an ideal capacitor is 10.sup.7 J/m.sup.3.
In addition to the means of storing energy taking advantage of a static electric field, there also is a means of storing energy using a static magnetic field, i.e., an energy storage device making use of a superconductive closed current. This method comprises forming a persistent current in a closed superconductive coil. In principle, the energy is stored in the form of a magnetic field having generated by the superconductive permanent current. The energy in a magnetic field is expressed by .mu.H.sup.2 =B.sup.2 /.mu., where, .mu. represents the permeability which is minimum in vacuum, i.e., 10.sup.-6 ; H represents the magnetic field; and B represents the magnetic flux density. The magnetic flux density, B, however, has an upper limit because under a magnetic flux density as high as 100 Tesla (T), the superconductor no longer maintains the superconductivity. In general, the superconductors can be used stably up to a flux density of 30 T. This signifies that the maximum achievable density of the accumulated energy is 10.sup.9 J/m.sup.3 which is considerably high for a physical means of storing energy. However, in the light of the present technological status, the superconductor applicable for this purpose requires cooling with liquid helium. This requirement can be fulfilled only with the construction of a gigantic cooling facility, and, moreover, the huge magnetic field having generated at the storage of energy requires shielding which accounts for a large volumetric portion of the whole installation. Thus, the energy storage device using the superconductive closed current is not yet to be applied in a commercial level.
In contrast to the physical means of storing energy described above, an energy storage means using flywheels has drawn much attention because of its simplicity, use of compact apparatuses, and the high density of stored energy achievable therewith.
In an apparatus using flywheels, in principle, the energy is conserved as a rotational energy (a type of kinetic energy) of the flywheels. The rotational energy is expressed by I .sup.2 /2 where I represents the moment of inertia and w represents the angular velocity of the flywheel. If the flywheel is a disk having a radius of r and a thickness of D, and made of a material having a density of p, the moment of inertia I thereof can be given by 2 .pi.pDr.sup.3 /3. Since the volume of a flywheel is expressed by .pi.r.sup.2 D the density of the accumulated energy becomes prw.sup.2 /3. Considering that commonly used substances have a density of about 10.sup.4 Kg/m.sup.3, e.g., 10.sup.3 Kg/cm.sup.3 for water and 2.times.10.sup.4 Kg/m.sup.3 in substances having maximum densities, it is more advantageous to increase other parameters than to seek materials having higher densities. From the equation above which represents the density of the accumulated energy, the density of the accumulated energy increases with increasing speed of rotation. When a flywheel having a radius of 1 m, which is made of a material whose density is 10.sup.4 Kg/m.sup.3, is rotated at a rate of 10.sup.5 rpm (which is nearly equal to 10.sup.3 rad/s), for example, the density of the accumulated energy can be obtained as 3.times.10.sup.9 J/m.sup.3. This density of the accumulated energy is extraordinarily large as compared with any other physical means of energy storage. If the rotational speed were to be increased by an order of magnitude, a density of the accumulated energy exceeding the upper limit of the stored energy density by a chemical means might be achieved. More advantageously, this means allows large energy discharge at one time, which can be never realized with a chemical means of storing energy.
It is also necessary to make environmental considerations in discussing the present day means for energy storage. Men have consumed fossil fuels such as coal and oil without taking particular notice of the environmental problems. The fossil fuels are advantageous in that they have considerably high density in the accumulated energy and that they allow simple handling. Thus, they are substantially free of problems in general use, and are widely used in many traffic and transportation means. In contrast to the fossil fuels having this advantage in handling, the fuels used in electric automobiles, hydrogen-fueled cars, and the like suffer inconvenience in handling and in the density of the accumulated energy. Thus, at the present, those cars are far from being commercial products.
However, as can be seen from the problem of global warming, the worldwide movement is headed to such which refrains from using fossil fuels, because the consumption of a fossil fuel signifies increasing emission of carbon dioxide into the atmosphere.
Furthermore, despite the importance as a basic material to support the present day industries, fossil fuels are subject to international affairs. This problem was made obvious in the recent Persian Gulf Crisis.
As an alternative energy source to replace fossil fuels, there can be mentioned nuclear power and solar photovoltaic cell power generation. However, the use of a nuclear power is disadvantageous in that there are difficulties in establishing and controlling nuclear power plants; moreover, it cannot be made so compact as to mount it on a car. Thus, nuclear power plants should be constructed at remote areas far from habitation, and the consumers use the electric power transmitted over a long distance. Furthermore, cars must store the electric energy in some manner and mount the stored energy inside the car to move freely. In the case of using solar photovoltaic energy, there is a drawback that energy cannot be supplied during night time and on cloudy days. This method therefore inevitably requires the energy to be stored.
At present, a study of converting the energy obtained by other means into artificial fuels (such as synthetic petroleum) is made. However, the production cost thereof is so expensive that is far from being practical.
As mentioned in the foregoing, it is strongly desired to store the energy having produced by nuclear power and solar photovoltaic cells in a form which allows men to use in convenience.
With respect to the conventional means of storing energy by the use of a flywheel, there have been problems concerning the structure. In the conventional flywheels, a bearing is incorporated to support the flywheel. The rotational speed of a flywheel is limited by this structure, and, in practice, a flywheel 1 m in diameter could not afford a rotational speed of 10.sup.5 r.p.m with a prior art technology.